In a medium-sized bowl, whisk together flour, baking powder, baking soda, and salt. Grate butter and add to flour mixture. Using your (clean) hands (or a pastry blender), mix the butter into the flour mixture until butter resembles small peas. Mix in the buttermilk until a shaggy dough just comes together.
Preheat oven to 450°F and lightly grease a baking sheet or cast iron skillet.
Turn mixture out onto a lightly floured surface and pat into a 1-inch thick square. Cut dough in half and stack one half on top of the other. Pat into a square. Repeat this process one more time. Cut dough into four square biscuits and transfer to the prepared baking sheet.
Bake for 10 to 12 minutes, or until tops are lightly golden brown. Remove from oven and brush tops with melted butter. Let cool slightly before serving.